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mongodb的锁机制

What type of locking does MongoDB use?

mongodb用的是什么类型的锁

MongoDB uses a readers-writer [1] lock that allows concurrent reads access to a database but gives exclusive access to a single write operation.

mongodb使用读写锁(共享-排他锁、多读单写锁),允许对数据库进行并发的读访问,并且对每一个写操作提供排他的访问。

When a read lock exists, many read operations may use this lock. However, when a write lock exists, a single write operation holds the lock exclusively, and no other read or write operations may share the lock.

当存在一个读锁时,很多读操作均可使用这个锁。当一个写锁出现,一个写操作排他的占有该锁,并且不被其他的读或者写操作共享。

Locks are “writer greedy,” which means writes have preference over reads. When both a read and write are waiting for a lock, MongoDB grants the lock to the write.

锁是偏向于写操作的,即相对于读操作,写操作有更高的优先权。当一个读和写操作同时在等待一个锁时,mongodb优先给写操作分配锁。

How granular are locks in MongoDB?

Changed in version 2.2.

mongodb中锁的粒度

Beginning with version 2.2, MongoDB implements locks on a per-database basis for most read and write operations. Some global operations, typically short lived operations involving multiple databases, still require a global “instance” wide lock. Before 2.2, there is only one “global” lock per mongod instance.

从2.2版本起,对大多数读写操作,mongodb实现了基于数据库的锁。全局操作,典型的比如涉及多个数据库的short lived操作,依旧会请求实例级别的全局锁,在2.2之前,每个mongodb实例只有一个全局锁。

For example, if you have six databases and one takes a write lock, the other five are still available for read and write.

例如,如果你有六个数据库,其中一个有写锁,其他的五个仍然可以进行读写。

How do I see the status of locks on my mongod instances?

我如何查看mongod实例的锁状态

For reporting on lock utilization information on locks, use any of the following methods:

db.serverStatus(),

db.currentOp(),

mongotop,

mongostat, and/or

the MongoDB Monitoring Service (MMS)

Specifically, the locks document in the output of serverStatus, or the locks field in the current operation reporting provides insight into the type of locks and amount of lock contention in your mongod instance.

To terminate an operation, use db.killOp().

Does a read or write operation ever yield the lock?

New in version 2.0.

读写操作会交出锁吗?

A read and write operations will yield their locks if the mongod receives a page fault or fetches data that is unlikely to be in memory. Yielding allows other operations that only need to access documents that are already in memory to complete while mongod loads documents into memory.

如果mongod接收到未命中的缓存页或者去取不在内存中的数据,读写操作将放弃他们的锁。这样就允许mongod进行其他只需要访问内存里的文档即可完成的操作,同时加载自身需要的数据到内存。

Additionally, write operations that affect multiple documents (i.e. update() with the multi parameter,) will yield periodically to allow read operations during these log write operations. Similarly, long running read locks will yield periodically to ensure that write operations have the opportunity to complete.

此外,多个文档上的写操作(比如带有multi操作的update),在日志写的时间内,会周期性地交出锁以容许读操作。类似的,长时间的读锁也会定期的退让以便写操作有机会完成。

Changed in version 2.2: The use of yielding expanded greatly in MongoDB 2.2. Including the “yield for page fault.” MongoDB tracks the contents of memory and predicts whether data is available before performing a read. If MongoDB predicts that the data is not in memory a read operation yields its lock while MongoDB loads the data to memory. Once data is available in memory, the read will reacquire the lock to completes the operation.

2.2版本极大地扩展了yielding,包括因为page fault的yielding。mongodb会跟踪内存并在执行一个读之前预判数据是否可用。如果数据不在内存,读操作即交出锁同时mongodb加载数据到内存。一旦数据到了内存,该读操作会再次搜索那个读锁完成此次操作。

[译文:http://hi.baidu.com/heelenyc/item/7e062b80b8610fceb17154a2]
[原文:http://docs.mongodb.org/manual/faq/concurrency/]

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